Add Ruffle support for Flash files.

Allow viewing Flash posts with the Ruffle emulator.

Known issues:

* Many flash files aren't fully supported.
* In development it sometimes spazzes out and starts triggering random
  keyboard shortcuts when you press any key. This doesn't happen with
  the browser extension.
* We have to put the .wasm file in the public/packs/js directory because
  Ruffle is hardcoded to search for it there.
* If you're running Nginx, you need to make sure you're serving the
  right MIME type for .wasm files or it won't work.
* We're using Some Random Guy's unofficial NPM package for Ruffle, since the
  Ruffle project doesn't publish an official package themselves. We
  should build our own package.

References:

* https://github.com/ruffle-rs/ruffle
* https://github.com/ruffle-rs/ruffle/wiki/Using-Ruffle#configure-webassembly-mime-type
* https://www.npmjs.com/package/ruffle-mirror

package.json

@@ -27,6 +27,7 @@
     "postcss-loader": "^5.0.0",
     "postcss-preset-env": "^6.7.0",
     "rails-erb-loader": "^5.5.0",
+    "ruffle-mirror": "^2021.3.31",
     "sass": "^1.32.5",
     "sass-loader": "^11.0.0",
     "spark-md5": "^3.0.0",